1. Field of the Invention
The present invention relates to a high frequency piezoelectric resonator and, more particularly, to a high frequency piezoelectric resonator that is adapted to suppress the occurrence of spurious.
2. Description of the Related Art
Following making higher the frequency, making higher the speed of data processing, and making larger the capacity, of communication apparatus, there has in recent years been a strong demand for making higher the frequency of a piezoelectric device that is used in each of these apparatus.
Development as to the increase in frequency of an AT cut crystal resonator has been being performed and, during this period of time, the frequency intended to be used therein has hitherto reached several hundreds of MHz. As well known, the waves in the vibration mode of the AT cut crystal resonator are in the thickness shear mode. Therefore, the frequency thereof is in inverse proportion to the thickness of the crystal plate. So, it is necessary to make thin the thickness of the crystal plate in order to make the frequency higher.
FIGS. 7(a) and 7(b) are views illustrating the construction of a conventional high frequency AT cut crystal resonator, FIG. 7(a) being a plan view and FIG. 7(b) being a sectional view. Part of a main surface of a crystal plate 21 is recessed using, for example, a photolithography technique and method of etching. The portion of the substrate 21 corresponding to this recess 22 is made to be an ultra-thin vibration portion. This portion of the plate 21 and a thick annular surrounding portion that retains the area around that vibration portion are formed integrally with each other. On one main surface (flat surface) of the crystal plate 21 there is disposed an electrode 23. From the electrode 23 there is extended toward the edge of the plate 21 a lead electrode 25. Further, on the other surface of the substrate 21, i.e., on the surface having therein the recess 22, there is formed an entire electrode 24.
It is to be noted that the energy trapping of the AT cut high frequency crystal resonator having a structure illustrated in FIG. 7, as well known, depends only upon the mass loading of the electrode 23 and not upon the mass loading of the entire electrode 24 on the other surface.
FIG. 8 is a view illustrating a measured example of the frequency spectrum of the AT cut high frequency crystal resonator having the structure illustrated in FIG. 7. The resonance frequency is set to be 156.6 MHz (the thickness of the crystal plate 21 is set to be approximately 10 xcexcm); the dimension in the X-axial direction of the electrode 23 is set to be 0.55 mm; the dimension in the Zxe2x80x2-axial direction is set to be 0.435 mm; and as the electrode 23 there was adhered a thin film of gold having a thickness of 600 xc3x85 on a backing layer of nickel having a thickness of 50 xc3x85. It is to be noted that the material of the film constituting the entire electrode 24 was also formed substantially in the same way.
As obvious from the frequency spectrum of FIG. 8, from the fundamental mode up to a large number of inharmonic modes fall within the range of the energy trapping mode. These modes, as seen, are sharply excited as spurious. Using this crystal resonator as one component of the oscillator, there is the possibility that the phenomenon of the frequency jump will occur.
As a method of analyzing a spurious mode in the thickness shear resonator such as that illustrated in FIG. 8, there is well known an energy trapping theory, which will briefly be explained below.
FIG. 9 is a sectional view illustrating the section of an ordinary AT cut crystal resonator. The diameter of a circular crystal plate 31 having a thickness of H is set to be 2b; the diameter of an electrode 32 that has been adhered thereto is set to be a; and the cutoff frequencies of the electrode 32 portion and the plate 31 portion are set to be f1 and f2, respectively. Generally, the plate-back xcex94, the energy trapping coefficient xcex6 and the normalized frequency "psgr" are expressed as follows.                     Δ        =                              (                                          f                2                            -                              f                1                                      )                                f            1                                              (        1        )                                ζ        =                              na            H                    ⁢                      Δ                                              (        2        )                                φ        =                              (                          f              -                              f                1                                      )                                (                                          f                2                            -                              f                1                                      )                                              (        3        )            
In the equation No. 2, n represents the order of the overtone mode. When the mode is the fundamental mode, n=1. FIG. 10 illustrates a frequency spectrum of the crystal resonator having the structure of FIG. 9, as determined by calculation, with the energy trapping coefficient xcex6 being plotted along the abscissa and the normalized frequency "psgr" being plotted along the ordinate. Generally, in order to design an resonator wherein spurious less occur, using the energy trapping coefficient xcex6=0.707 that is immediately before the symmetrical 1st mode S1 starts to be trapped is admitted as being preferable.
Here, FIG. 10 illustrates the results that have been obtained by the calculation that has been performed on the premise that the substrate be an isotropic elastic body. However, if applying this method of determining the frequency spectrum to an anisotropic piezoelectric plate such as a crystal, it is well known that it is sufficient to multiply each length by an anisotropic constant to thereby correct this length. For instance, with respect to the thickness twist mode and the thickness shear mode of the crystal resonator, their respective optimum energy trapping coefficients xcex6 are said to be 2.4 and 2.8.
Determining the energy trapping coefficient xcex6 of the high frequency crystal resonator illustrated in FIG. 8 according to the equation (2) as a trial, the xcex6=5.7. It is seen that this value is the one that is much greater than the optimum value. Therefore, as stated above, a high order of vibration mode also becomes an energy-confining mode, whereby a large number of spurious are sharply excited.
On the other hand, in order to improve the spurious characteristic of the AT cut crystal resonator, a piezoelectric resonator that can somewhat arbitrarily control the frequency of the generated spurious regardless of the length, film thickness, etc. of the electrode has been proposed in Japanese Patent Application Laid-Open Nos. Hei-9-27729 and Hei-9-93076.
FIG. 11(a) is a plan view illustrating a crystal resonator that is as proposed above and FIG. 11(b) is a sectional view thereof. On both surfaces of the central parts of a crystal plate 41 there are disposed main electrodes 42a and 42b. Simultaneously, second electrodes 44a and 44b are disposed so as to surround the peripheral edges of those electrodes 42a and 42b and with a gap between the both. The respective cutoff frequencies of the main electrodes 42a, 42b, gap portion, and second electrodes 44a, 44b are set to be f1, f2, and f3, as illustrated in FIG. 11(b). Here, the thickness of the electrode films are set so that the relationship among the cutoff frequencies of f1 less than f3 less than f2 holds true among the cutoff frequencies.
Also, the second amount xcex942 of decrease in frequency and the depth xcexd of the gap (hereinafter referred to as xe2x80x9ca groove depthxe2x80x9d) are respectively defined as follows.                               Δ          ⁢                      xe2x80x83                    ⁢          2                =                              (                                          f                3                            -                              f                1                                      )                                f            1                                              (        4        )                                v        =                              (                                          f                2                            -                              f                3                                      )                                (                                          f                3                            -                              f                1                                      )                                              (        5        )            
According to the above official gazettes, whatever value the mass loading (the film thickness) of the main electrode 42a, 42b is set to be at, only if appropriately setting the mass loading (the film thickness) of the second electrode 44a, 44b correspondingly, it becomes possible to decrease the amount xcex942 of decrease in frequency, which participates in the energy trapping coefficient xcex6, down to a desired value. The above official gazettes describe that therefore it becomes possible to easily control the energy trapping coefficient xcex6 and hence to manufacture a crystal resonator the spurious of that are less. Further, the above official gazettes describe that, by aptly setting the groove depth xcexd and the ratios q/a and b/a of the distance q and b of the main electrode 42a, 42b with respect to the both edges of the second electrode 44a, 44b, it is possible to somewhat arbitrarily control the value of the energy trapping coefficient xcex6 permitting the generation of a symmetric high-order mode and anti-symmetric mode. Therefore, if adopting this proposal, it results that only desired main vibration waves can be excited with a high level of power without making an inharmonic mode trapped mode.
However, applying the above proposal to a high frequency 150-MHz band crystal resonator and determining the plate-back xcex942 using the electrode configuration of FIG. 8 with the energy trapping coefficient xcex6 being set to be 0.521, this plate-back xcex942 becomes xcex942=0.00144. If using a thin film of gold for both the main electrode and the second electrode, the difference in film thickness between the main electrode and the second electrode becomes 21.8 xc3x85. To realize this difference in film thickness, a precision of 10% or so becomes necessary with respect to the value of the difference. This value goes beyond a limit of control that is possible with an ordinary evaporating or sputter apparatus. Also, even when having used the value, which is immediately before the S1 mode is trapped, as the energy trapping coefficient xcex6, recognizing the generation of the oblique-symmetrical A0 mode, the difference in film thickness between the main electrode and the second electrode is as very small as 40 xc3x85. Controlling such a small film thickness difference with a precision of 10% or so is virtually impossible. Even if a film-forming apparatus enabling such control to be made has been developed, there was the problem that such an apparatus became very expensive.
Also, in case that forming a AT cut high frequency crystal resonator such as that illustrated in FIG. 7 using method of etching, the crystal plate prior to etching is etched until its thickness becomes 10 xcexcm while maintaining the surface configuration of the substrate, for example, 80 xcexcm thick as it is. Therefore, the unevenness in the flatness and the parallelism, which did not become very problematic with the substrate 80 xcexcm thick, becomes relatively very greatly problematic when the same is viewed from the etched plate that has become 10 xcexcm thick. Resultantly, anti-symmetrical modes A0 and A1 of vibration, which in an ordinary crystal resonator didn""t become problematic due to cancellation of the electric charges, appear as vigorous spurious in the thin portion which vibrates. For this reason, the following problem arose. Namely, as the energy trapping coefficient xcex6 that is optimum to suppressing the occurrence of spurious, the value that is immediately before the symmetrical 1st mode S1 starts to be trapped is not adequate. In other words, a sufficiently small value of energy trapping coefficient xcex6 must be used so that the anti-symmetric mode A0 does not become trapped.
The present invention has been made in order to solve the above-described problems and has an object to provide an AT cut high frequency crystal resonator wherein the occurrence of spurious is less.
To attain the above object, according to the first aspect of the invention, there is provided a high frequency piezoelectric resonator, the piezoelectric resonator including a piezoelectric plate having disposed on its main surfaces, respectively, mutually opposing main electrodes for the excitation, a pair of second electrodes being each disposed surrounding the peripheral edge of its corresponding main electrode with a gap in between, wherein the material of the main electrode and the material of the second electrode are different from each other.
According to the second aspect of the invention, there is provided a high frequency piezoelectric resonator according to the first aspect, wherein the density of the material of the second electrode is made lower than that of the main electrode.
According to the third aspect of the invention, there is provided a high frequency piezoelectric resonator according to the first or second aspect, wherein the piezoelectric plate is made a piezoelectric plate having formed therein a recess.
According to the fourth aspect of the invention, there is provided a high frequency piezoelectric resonator according to the first to third aspects, wherein the configuration of the main electrode is made elliptic.
According to the fifth aspect of the invention, there is provided a high frequency piezoelectric resonator including a piezoelectric plate, one main surface of the piezoelectric plate being recessed to thereby form a thin portion therein, the main surface opposing the recess corresponding to the thin portion having formed thereon at its central portion a convex portion, the convex portion having formed thereon a main electrode for the excitation, a lead electrode being extended from the main electrode toward an edge of the plate, a second electrode being so provided as to surround the main electrode and the lead electrode with a gap in between, the piezoelectric plate having applied on a recess side thereof an entire electrode.
According to the sixth aspect of the invention, there is provided a high frequency piezoelectric resonator according to the fifth aspect, wherein the convex portion is made elliptic.
According to the seventh aspect of the invention, there is provided a high frequency piezoelectric resonator according to the first to sixth aspects, wherein the second electrode is divided into a plurality of portions; and adjustment of frequency is performed of these electrode portions.